1. Field of the Invention
The present invention relates generally to computer-aided methods and systems for manufacturing products in a high volume, automated, continuous process and, more particularly, to improved methods and apparatus for automated retrieval, alignment, placement and securement of singulated bare semiconductor dice within preformed packages for testing and burn-in, followed by optional subsequent removal of the dice from the packages.
2. State of the Art
Integrated circuit devices are well-known in the prior art. Such devices, or so-called "dice," may include a large number of active semiconductor components (such as diodes, transistors) in combination with (e.g., in one or more circuits with) various passive components (such as capacitors, resistors), all residing on a "chip" or die of silicon or, less typically, gallium arsenide. The combination of components results in a semiconductor or integrated circuit die which performs one or more specific functions, such as a microprocessor die or a memory die, as exemplified by ROM, PROM, EPROM, EEPROM, DRAM and SRAM.
Such dice are normally designed to be supported or carried in a package having a plurality of externally-accessible pins or leads, to which terminals such as bond pads on the die are electrically connected within the package to access other electronic components employed in combination with the die. A package provides mechanical support and protection for the die, may serve as a heat sink, and is normally square or rectangular in shape. The packages typically comprise a filled polymer compound transfer molded about a die wire-bonded or otherwise electrically connected and physically supported by a lead frame structure, or a two-piece preformed ceramic package to which the die is physically and electrically connected before the package lid is secured. Met packages are also used, although generally in small quantities and for so-called "military spec" applications.
Packaging defective dice or unknown bad dice (UBD) which are packaged, tested and then scrapped after proven defective in post-packaging testing is inefficient and costly. Accordingly, the bare dice are often tested for continuity during the die fabrication process and before packaging. Such testing may be and has been accomplished by placing bare die in temporary packages having terminals aligned with the terminals (bond pads) of the die to provide electrical access to the devices on the die and subjecting the die via the assembled package to extensive testing, which includes burn-in and discrete testing. Exemplary state-of-the-art fixtures and temporary packages for die testing are disclosed in U.S. Pat. Nos. 5,367,253 and 5,519,332 (to some of the inventors named herein); 5,448,165; 5,475,317; 5,468,157; 5,468,158; 5,483,174; 5,451,165; 5,479,105; 5,088,190; and 5,073,117. U.S. Pat. Nos. 5,367,253 and 5,519,332, assigned to the assignee of the present application, are each hereby incorporated herein for al purposes by this reference.
Discrete testing includes testing the die devices for speed and for errors which may occur after fabrication and after burn-in. Burn-in testing is conducted at elevated potentials and for a prolonged period of time, typically 24 hours, at varying and reduced and elevated temperatures such as -15.degree. C. to 125.degree. C. to accelerate failure mechanisms such that die devices which have the potential to prematurely fail during normal operation can be identified and eliminated. Dice which survive discrete testing and burn-in are termed "known good die," or KGD.
Failure of one die on a multi-chip module (MCM), including a so-called single in-line memory module (SIMM), compromises performance of the entire module or, if identified after assembly but before shipment to the customer, at the least initiates a relatively costly and time-consuming rework process to replace the bad die if the entire MCM is not to be scrapped. Even if individual die yield is relatively high, the combination of such dice in an MCM nonetheless produces an abysmal module yield. For example, if a particular MCM design includes twenty (20) dice with an average "good die" yield rate of 97.3%, the overall yield rate would be predicted to be a dismal 57.3%, which is not commercially viable. Moreover, subjecting the printed circuit or other die carrier of the MCM to burn-in may not be desirable as causing unnecessary stress on elements of the MCM other than the die. Therefore, employing KGD in an MCM is perceived as an optimum way to fabricate high-reliability multi-die products.
However, while desirable, testing bare, unpackaged dice requires a significant amount of handling. The temporary package must not only be compatible with test and burn-in procedures, but must also physically secure and electrically access the die without damaging the die at the bond pads or elsewhere. Similarly, assembly of the die with the package and disassembly after testing must be effected without die damage. The small size of the die itself and minute pitch (spacing) of the bond pads of the die, as well as the fragile nature of the thin bond pads and protective layer covering devices and circuit elements on the active surface of the die, makes a somewhat complex task extremely delicate. Performing these operations at high speeds with requisite accuracy and repeatability has proven beyond the capabilities of the state of the art.
Bond pads are discrete conductive areas on the active face of the die which are used for connecting the internal die circuitry to the conductors of the package. Accurate positioning of the die within the temporary package is therefore critical since alignment of the die bond pads relative to the contacts of the temporary package electrical conductors must be effected in order to subject the die to testing.
Precising die packaging includes mechanically locating a component in a precise position or placement. Various "precising" methods for this purpose are known in the art. However, there have been several problems associated with such precising methods and systems. For example, it has proven difficult to position the die bond pads in electrical contact with temporary package electrical contacts in an accurate and consistent manner so as to facilitate a repeatable, high volume, continuous assembly process of dice within temporary packages. Another disadvantage associated with prior art equipment and processes is that the die is often destroyed or damaged upon contact with the temporary package, lowering product yield and profit margins. Accurate, repeatable positioning placement and securement of the die in the temporary package is thus critical to providing acceptable KGD qualification on a commercial basis.
One attempt to overcome the problems associated with the prior art has been to precise dice and packages by mechanical fixturing. However, assembly tolerances used in mechanical fixturing techniques are often insufficiently fine to prevent improper alignment. Mechanical fixturing also leads to damage of the die or temporary package. While such techniques have proven useful in improving the accuracy and reliability of the die placement, these techniques do not enable dice to be precisely positioned within temporary packages in a manner that allows production efficiencies capable of supporting large volume operations.
Other systems for alignment and, optionally, placement of various bare and packaged dice are also known in the art. See, for example, U.S. Pat. Nos. 4,526,646; 4,543,659; 4,736,437; 5,052,606; 5,059,559; 5,113,565; 5,123,823; 5,145,099; 5,238,174; 5,288,698; 5,463,227; and 5,471,310 for vision-based systems. A commercially available vision-based aligner bonder for flip chip bonding, offered by Research Devices of Piscataway, N.J., has also been modified by the assignee of the present invention for manual alignment of bare dice with the electrical contacts of a temporary package employed in KGD qualification. It is believed that certain aspects of the commercial Research Devices system may be disclosed in U.S. Pat. No. 4,899,921. A description of the modified Research Devices system appears in the aforementioned U.S. Pat. No. 5,519,332, assigned to the assignee of the present invention and incorporated herein for all purposes by this reference. A discussion of vision systems' potential applications in the semiconductor industry and associated problems appears in "A Vision of Vision in the Gigabit Era," SEMICONDUCTOR INTERNATIONAL, June 1993, pp. 120-122, 124.
While the foregoing mechanical and visual alignment systems, with ancillary mechanisms for die handling, have achieved some success in their intended applications, to the inventors' knowledge there exists no fully-automated bare die and package assembly and disassembly system capable of accurate and repeatable operation at a speed making KGD qualification or characterization commercially viable for use as a matter of course in the die fabrication process.
Accordingly, there remains a long-felt need in the semiconductor industry to provide for improved methods and apparatus for assembling dice to be tested with temporary packages (and subsequently disassembling the dice from the packages) in a high volume, cost-efficient and reliable manner. Toward that end, it is essential that the semiconductor or integrated circuit die be positioned and secured within the temporary packages in an automated manner such that die bond pads are aligned with and suitably biased toward temporary package electrical contacts without physical damage to the die structure.